1. Field of the Invention
The invention relates to a sealing device and, more particularly, to a sealing device that hermetically seals a gap between a first member and a second member reciprocally movable relative to the first member to seal liquid supplied into a pressurizing chamber surrounded by the first member and the second member.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 2006-207820 (JP-A-2006-207820), Japanese Patent Application Publication No. 2006-46550 (JP-A-2006-46550) and Japanese Patent Application Publication No. 2005-273782 (JP-A-2005-273782) each describe a technique related to a sealed housing.
JP-A-2006-207820 suggests a technique for achieving high hermeticity and low sliding resistance in such a manner that, in a lip that seals an oil chamber, recesses or protrusions that define recesses are formed on a sliding surface to store hydraulic oil. JP-A-2006-46550 suggests a technique for reducing a sliding resistance in such a manner that a protrusion is provided on a sliding surface to hold hydraulic oil. JP-A-2005-273782 suggests a piston equipped with a lip that holds lubricating oil for applying lubricating oil over the entire contact surface at the time when the piston is assembled to a clutch drum.
In a frictional engagement device, such as a clutch and a brake, installed in a vehicle automatic transmission, hydraulic pressure is applied to a piston to press a plurality of frictional engagement elements to engage the frictional engagement elements, thus selectively activating respective gears. When a specific shift of the automatic transmission is carried out, two different frictional engagement devices may respectively engage and release at the same time (so-called clutch-to-clutch shift).
In the clutch-to-clutch shift, if the progress of engagement of the engaging-side frictional engagement device delays from the progress of release of the releasing-side frictional engagement device, there occurs a situation that both the releasing-side and engaging-side frictional engagement devices are not sufficiently engaged. As a result, an engine rotational speed increases, and a phenomenon, so-called “engine racing”, occurs. This makes it difficult to achieve a desirable shift feel. For example, when air is trapped into an oil chamber by which the piston of the frictional engagement device is controlled for movement, air is compressed to delay an increase in hydraulic pressure when the hydraulic pressure in the oil chamber is increased. This may cause engine racing.
Even when the engine racing occurs because of a delay of rising of the hydraulic pressure in the oil chamber, a command value may be fed back through learning control. Specifically, delaying hydraulic control over the releasing-side frictional engagement device or advancing hydraulic control over the engaging-side frictional engagement device is, for example, carried out as the learning control. In this manner, it is possible to suppress occurrence of engine racing from the next shift.
However, the amount of air trapped in the oil chamber is not constant, and is variable. As the amount of air in the oil chamber increases, rising of hydraulic pressure delays against a command value to cause engine racing. In addition, as air suddenly comes out of the oil chamber and then the amount of air trapped in the oil chamber reduces, the progress of engagement of the engaging-side frictional engagement device advances relative to the progress of release of the releasing-side frictional engagement device. As a result, there occurs a state (tie-up) in which a plurality of frictional engagement devices for different gears are engaged at the same time. This causes a shift shock that is dependent on a difference in rotational speed between frictional engagement elements.
It is difficult to recognize when air is trapped in the oil chamber. Trapped air usually accumulates in the oil chamber, but the air may suddenly come out of the oil chamber. Thus, it is difficult to predict the amount of air in the oil chamber. When a command value for learning control is set in a state where air is trapped in the oil chamber, there is a problem that controllability deteriorates because of variations in the amount of air trapped in the oil chamber, and, as a result, engine racing or a shift shock occurs. However, the above related techniques have not suggested a solution to the above problem.